Automatic transmissions of the type addressed by this invention include several fluid operated torque transmitting devices, referred to herein as clutches, which are automatically engaged and disengaged according to a predefined pattern to establish different speed ratios between input and output shafts of the transmission. The input shaft is coupled to an internal combustion engine through a fluid coupling, such as a torque converter, and the output shaft is mechanically connected to drive one or more vehicle wheels.
The various speed ratios of the transmission are typically defined in terms of the ratio Ni/No, where Ni is the input shaft speed and No is the output shaft speed. Speed ratios having a relatively high numerical value provide a relatively low output speed and are generally referred to as lower speed ratios; speed ratios having a relatively low numerical value provide a relatively high output speed and are generally referred to as upper speed ratios. Accordingly, shifts from a given speed ratio to a lower speed ratio are referred to as downshifts, while shifts from a given speed ratio to a higher speed ratio are referred to as upshifts.
In most transmissions, ratio shifting is carried out by directing the fluid pressure output of a driven pump, referred to as line pressure, to the various clutches of the transmission in accordance with a predefined pattern. To this end, the transmission includes hydraulic or electro-hydraulic elements, such as shift valves and accumulators activated in response to vehicle operating parameters.
Certain transmissions also include electrical elements for controlling the line pressure regulation in accordance with a scheduled pressure obtained from a table look-up. The scheduled pressure is generally speed and load (torque) dependent, and operates not only to maintain adequate torque capacity in engaged clutches, but also to control clutch engagement rate during shifting. In such transmissions, an adaptive trim or correction of the scheduled pressure can be employed as a means of compensating for variability associated with part-to-part tolerances, wear, etc. One such control, set forth in U.S. Pat. No. 4,283,970 to Vukovich, issued Aug. 18, 1981, and assigned to the assignee of the present invention, develops an adaptive correction of the scheduled line pressure based on a deviation of the actual shift time from a desired shift time characteristic of high quality shift feel. Also see U.S. Pat. No. 4,653,350 to Downs et al., issued Mar. 31, 1987, and assigned to the assignee of the present invention.
According to the Vukovich patent, the adaptive correction is stored in a correction register, which maintains a single cumulative correction for the normally scheduled line pressure. In practice, a number of correction registers for various ranges of engine throttle setting (or input torque) have been employed. According to the Downs patent, the adaptive correction updates are apportioned between low and high torque data points of a correction table, and the adaptive correction for intermediate torque levels is obtained by interpolation.